So far it has been proposed that a dye carrier paper having an ink containing sublimation dye is superposed at its one surface to a printing paper having surface treatment, the dye carrier paper is heated at another side by a thermal print head and the dye in the ink is sublimated and thermally transferred to the printing paper whereby to obtain a color hard copy. On the surface of the hard copy thus made is adhered by melt a cover film as a surface protective layer so that when a hand, for example touches the surface of the hard copy, color of the hard copy can be prevented from being faded by oils and the like carried from the hand to the surface of the hard copy.
FIG. 1 shows an overall structure of an example of a prior art cover film for a sublimation transfer type color hard copy.
In FIG. 1, reference numeral 1 designates a cover material and this cover material 1 is supported by a base material 2 to form the whole of a cover film 3 for a sublimation transfer type color hard copy. The cover material 1 is made of polyester resin (VYLON #200, manufactured by Toyobo Co., Ltd.) of 5 .mu.m thickness having melt bonding property. The base material 2 is made of polyester film of 12 .mu.m thickness, and the cover material 1 is coated on the surface of the base material 2.
FIG. 2 shows another example of the prior art cover film for a sublimation transfer type color hard copy.
In FIG. 2, like parts corresponding to those in FIG. 1 are marked with the same references and will not be described in detail.
In this example, on a base material 2 made of polyester film having a thickness of 12 .mu.m is formed a first layer of 1a cover material of about 5 .mu.m thickness made of cellulose acetate butylate resin (manufactured by Sumitomo Bayer Urethane Co., Ltd.) on which a second layer 1b of cover material of about 5 .mu.m thickness made of polyester resin having melt bonding property is further formed. By the way, although in the color hard copy formed by the transfer of dye the dye is partly diffused and colored in the surface treated layer on the surface of the printing paper, most of the transferred dye is still coagulated so that the dye can not present its original color sufficiently. A dye having a relatively satisfactory coloring property suitable for sublimation transfer printing is frequently found out in dispersing dyes, basic dyes and solvent dyes. However, when such dye is used in the dye carrier paper, most of such dye is limited to the dispersing dye and the kinds of such dye are limited to several tens in practice. When the colors of the dye are classified into three primary colors, cyan, magenta and yellow in accordance with subtractive color mixture, the kinds of dye suitable for the color hard copy are further limited for each primary color. When the hues of these dyes transferred on the printing paper are observed, particularly the magenta color becomes reddish. As a result, of red, green and blue colors as the mixed colors of three primary colors of the subtractive color mixture, particularly red color presents a yellowish color, namely, a color close to orange color. For this reason, it was desired that the coloring of reddish dye was arbitrarily controlled to shift bluish side so as to present an optimum magenta color suitable for mixed colors.
In this connection, the present inventor got the following experimental results regarding the coloring of reddish dye in the case where the prior art cover films of sublimation transfer type shown in FIGS. 1 and 2 were used.
In the first experiment, a printing paper for a sublimation transfer type color hard copy was formed such that a coating composition formed of 24 parts by weight of internally plasticized saturated polyester resin (VYLON #200, manufactured by Toyobo Co., Ltd.), 6 parts by weight of ultra fine particle silica (NIPSIL E220A, manufactured by Nippon Silica Industrial Co., Ltd.) and 70 parts by weight of methyl ethyl ketone solvent was coated on one surface of a best quality paper having an area weight of 170 g/m.sup.2 so as to have a coating weight of about 5 g/m.sup.2 after being dried. On the other hand, a dye carrier paper to obtain a print sample was formed such that an ink for magenta color formed of 6 parts by weight of anthraquinone-series reddish dispersing dye (PTR 63, manufactured by Mitsubishi Chemical Industries Co., Ltd.), 6 parts by weight of ethyl cellulose and 88 parts by weight of isopropyl alcohol solvent was coated on a thin paper having an area weight of 40 g/m.sup.2 by using a gravure coater so as to have a coating weight of 5 g/m.sup.2 after being dried. Then, a picture image of magenta color was printed on the printing paper by heating from the back of the dye carrier paper with a thermal print head heated at a temperature of about 300.degree. C. under the condition that the printing paper was in contact with the ink coated surface of the dye carrier paper, and a sample of a sublimation transfer type color hard copy was obtained. Then, the cover film B in the above example shown in FIG. 1 was pressedly attached on the hard copy by using a hot plate heated at 150.degree. C. thus a color print being obtained. In this experiment, since the change of red color was expressed as 0.456 by a chromaticity coordinate X value of CIE (Commission International de L'Eclairage) standard colorimetric system, the coloring of reddish dye had to be controlled to properly shift bluish side so as to present the magenta color of high color saturation. The X value of the CIE standard colorimetric system means that the larger X value increases the degree of red color, while the smaller X value increases the degree of blue color.
In the second experiment, after the cover film 3 in the example shown in FIG. 2 was formed on the sample hard copy formed in the first experimental example by a hot press, the base material 2 made of polyester film was released therefrom so that a sublimation transfer type cover film layer was formed on the printing paper thus a color print being obtained. Since the change of red color in the second experiment was expressed as 0.462 by a chromaticity coordinate X value of CIE standard colorimetric system, the coloring of reddish dye had to be controlled to properly become bluish so as to present the magenta color of high color saturation.
The printing paper suitable for the sublimation transfer color copy is generally formed such that a small amount of resin such as polyester resin, epoxy resin, nylon resin, polyvinyl pyrrolidone resin, acetate cellulose or the like which can easily adsorb and diffuse a dye is coated on the surface of the printing paper.
However, these resins are all colored slightly yellowish and as the resin ages, such resin is apt to be colored yellowish easily. In addition, a paper on which such coating composition is coated is generally colored yellowish and the degree of yellowish color is raised more by heat generated from the coating composition when it is dried or hardened. As a result, the picture image printed on the printing paper is caused to lose its clearness, and in the case of the colored layer formed by a dye of high transparency, its hue itself is caused to be displaced from its original hue. On the other hand, it was necessary that a paint of high whiteness such as titanium oxide or the like was added into the coating composition to increase whiteness degree. If necessary, a phosphor whitener or a small amount of blue color dye must be added thereinto so that a paper used to form thereon a picture image inevitably became expensive.
Alternatively, after the dye carrier paper formed by using the sublimation dye and the printing paper subjected to the surface treatment were superposed on each other with a predetermined positional relation, the sublimation dye sublimated by heating mainly from the back of the dye carrier paper was transferred on the coating layer on the surface of the printing paper thus the picture image being formed. Although the dye transferred on the printing paper was partly diffused into the coating composition layer and colored, most of the dye was coagulated and merely transferred thereon. Therefore, the dye could not present its original color satisfactorily. In addition, the dye is easily faded by oils and the like carried from, for example, hands and is also easily faded by ultraviolet rays contained in natural light and oxygen in the air.
The third and fourth experiments regarding the prior art technique in which the conventional cover film for a color hard copy printing paper was melted by heat on the printing paper will be described in detail.
The third experiment using the prior art technique will be described as follows. An aqueous solution for surface treatment of the printing paper formed of polyvinyl pyrrolidone (manufactured by General Aniline & Film Corporation), polyester resin (VYLONAL MD-1200, manufactured by Toyobo Co., Ltd.) and calcium carbonate (manufactured by Nitto Funka Kogyo Kabushiki Kaisha) was coated on a best quality paper of area weight of 170 g/m.sup.2 which was not particularly bleached for the purpose of forming a picture image to have a coating amount of about 5 g/m.sup.2 and thus a printing paper of whiteness degree of 91.2 was obtained. Then, as shown in FIG. 1, the cover material 1 of 5 .mu.m thickness made of linear polyester resin (VYLON #200, manufactured by Toyobo Co., Ltd.) was formed on the base material 2 of 15 .mu.m thickness made of polyester film to form the cover film 3 for a sublimation transfer type hard copy. This cover film 3 and the above printing paper were made in contact with each other and pressed to each other by hot press at about 150.degree. C. and the cover film 3 in this example was melted by heat on the printing paper thus a color print sample being obtained. In this experiment, if the whiteness degree of pure white was taken as 100, the whiteness degree of the color print sample was 91.2. Moreover, according to the CIE standard colorimetric system chromaticity coordinate (the achromaticity axes by Macbeth Corporation were presented as x=0.310 and y=0.316), x=0.317 and y=0.327 were obtained. Thus, similarly to the case in which the cover film was not melted by heat on the printing paper, the whiteness degree and the clearness were not satisfactory.
The fourth experiment using the prior art technique will be described as follows. An ink for cyan color was formed of 6 parts by weight of sublimation cyan dye (KAYALON FAST BLUE-BR, manufactured by Nippon Kayaku Co., Ltd.), 6 parts by weight of ethyl cellulose and 88 parts by weight of isopropyl alcohol solvent. This ink for cyan color was coated on the surface of a paper having an area weight of 40 g/m.sup.2 by using a gravure coater so as to have a coating amount of 5 g/m.sup.2 after being dried and thus a dye carrier paper was formed. Then, this dye carrier paper and a printing paper prepared similarly to the third experiment using the prior art technique were superposed on each other, pressed for 5 seconds from the side of the dye carrier paper by hot press heated at 200.degree. C., and the sublimation dye was sublimated from the dye carrier paper, thus a picture image being transferred on the printing paper. Then, the cover film 3 prepared by the above third experiment using the prior art technique was superposed on the color print and heated to form a colored color print.
In this experiment, this color print was exposed to ultraviolet rays for 100 hours by using a fade meter (standard ultraviolet ray long life fade meter, manufactured by Suga Test Instruments Co., Ltd.) and thereafter its tenebrescence was measured by Macbeth reflection density meter RD-514 on the basis of the change of cyan concentration. The cyan concentration of the color print before being exposed to the ultraviolet rays was 1.24 and the cyan concentration thereof after being exposed to the ultraviolet ray was 1.11, resulting in color fading ratio of 1.24. As a result, something had to be done to prevent the color from being faded.
In view of the above aspects, the present invention is to provide cover film for a sublimation transfer type hard copy capable of producing magenta color of high color saturation by arbitrarily controlling the coloring of reddish dye to be shifted to the blue side and which can protect the surface of the printing paper.
Also, the present invention is to provide a cover film for a sublimation transfer type hard copy in which a dye on a printing paper is diffused and colored to increase a whiteness degree of a portion which is not colored so that a picture image is made clear and which can prevent the color of dye from being faded.